September 14, 2011

Nitrogen Tire Inflation and Temperature Change

With cool fall nights fast approaching here in the Northeast, I thought it would be valuable to post an old submission that covers nitrogen tire inflation and temperature change. I have heard from numerous readers that they find this post to be very valuable, so here goes.

There is no significant difference in expansion and contraction characteristics of nitrogen compared to air when moisture is absent. Expansion or contraction of either gas, in relation to temperature change, occurs to a similar extent over the commonly encountered pressure and temperature ranges releveant to the discussion of tire inflation. There is no practical difference as long as the gases are dry in a fixed volume container such as a tire.

On the other hand, water is usually present in the case of conventional compressed air where dewpoints can be as high as 70F, compared with -50F for nitrogen. At lower temperatures, water (as a liquid) occupies very little volume. However, as temperature increases liquid water vaporizes to become a gas and its volume expands. This causes total tire pressure to be higher than it would be in the case of a dry gas, such as nitrogen. Thus the presence of water in a tire contributes to wild pressure variations as temperatures change.

The pressure in nitrogen filled tires will change when the temperature changes, just as it does with compressed air filled tires because nitrogen responds to changes in ambient temperature in a similar manner. For example, when your vehicle is parked, it will lose 1.9% of its pressure for every 10F change in temperature. These calculations are based on the ideal gas law. If a tire is filled to 32 psig at a temperature of 75F, and the outdoor temperature is 35F, the tire pressure will drop to 29 psig. These fluctuations will occur as the temperature rises and falls, regardless of the inflation gas. Fortunately, tire manufacturers are readily aware of these parameters and set their cold inflation pressures accordingly.

So, the bottom line is that you will still see pressure changes with nitrogen. But they should be more consistent and run cooler than if they were filled with compressed air.